In my experiences, thrust blocks are actually quite common even still today. The method depends on the type of pipe material, size and site constraints. It also depends on the pressures expected (remember to count for surge) and the type of bend. Our company has a standard table and detail...
I don't think there is a general standard. The 18" vertical clearance comes from the 10 states standards for sewer and water lines. You should check your state's requirements. If there is no code you're ok on that distance. However, good engineering judgment would be to make the sanitary line...
